F. m. stereophonic multiplex receiver having a single volume control for adjusting the magnitude of the signals presented to the stereo detector and the output materixing means



Oct. 6, 1964 A. COTSWORTH 3,

F.M. STEREOPHONIC MULTIPLEX RECEIVER HAVING A SINGLE VOLUME CONTROL FORADJUSTING THE MAGNITUDE OF THE SIGNALS PRESENTED TO THE STEREO DETECTORAND THE OUTPUT MATRIXING MEANS Filed Aug. 1, 1962 Filter (A-B)S //5lPhase f I "JV- I E) I United States Patent 0 3,152,224 EM. STEREQPHQNTCMULTIPLEX PECEIVER HAVING A SINGLE VGLUME CONTROL FGR ADEUSTING THEMAGNHTUDE Gir THE SIGNALS PRESENTED Til THE STAB-tilt) DETECTQR AND THEOUTPUT MATPIG lviEANS Aihert Cotsworth, Oak Park, Ill., assignor toZenith Radio Corporation, Chicago, 111., a corporation of Delaware FiledAug. 1, 1962, Ser. No. 213,919 1 Claim. (ill. 17915) The presentinvention relates to radio receivers which may be employed in thereception of stereophonic broadcast programs. More particularly, itconcerns itself with loudness control circuitry for varying the soundlevel of the reproduced program.

To achieve stereophonic reproduction, it is essential that thereproducing system comprise two separate audio channels drivingappropriately spaced loudspeaker systerns. In the usual case, thereceiver for the reproduction of stereo programs is constructed with aprincipal chassis containing one of the speakers and all of thereceiving and amplification circuitry necessary to produce thestereophonic program signals. There is an associated remote speakerpositioned for optimum stereo effect and cable connected to theprincipal channel to be driven therefrom. In keeping with truestereophonic reproduction it is necessary that the balance or relativesound level between the two speakers be kept constant when the programvolume is varied. It is customary to employ concentric, simultaneouslyactuated potentiometers to adjust the gain of the two amplifier channelsof the receiver to vary the level of the reproduced sound but a trackingerror may be encountered. This error, which may be attributed to thepotentiometer construction and to tolerance variations impairs thebalance of the reproduced programs and is especially noticeable todiscriminating listeners.

In one prior arrangement concentrically mounted p0- tentiometers mecoupled between the stereo detector and the audio amplifiers of thesignal channels with their actuating shafts coupled together by way of aclutch mechanism. Two concentric control knobs are provided with theouter knob causing both potentiometers to track as a loudnessadjustment. Adjustment of the balance of the two audio channels isachieved by rotating the inner control knob and holding the outer knobfixed. However, because of tracking errors associated with theconcentric otentiometers, the balance may vary when a loudnessadjustment is subsequently made.

In another arrangement, the stereo detector supplies left and rightstereo signals to separate audio amplifiers by way of individualpotentiometers which are concentrically mounted but, in contrast to thefirst arrangement, their control shafts are rigidly coupled together.Between these potentiometers and the audio amplifiers are individualbalance potentiometers which are so arranged that when actuated, thesignal supplied to one amplifier varies inversely to the signal appliedto the other. However, after the channels have been balanced, error maybe introduced when the loudness control is varied, again, because oftracking dificulty.

Still another arrangement, which also employs concentrically mounted andsimultaneously actuated loudness potentiometers for adjusting themagnitude of the left and right stereo signals, has a singlepotentiometer, poistioned between the stereo detector and amplifier inone channel, to vary the balance. Here again, tracking errors mayintroduce unbalance when a loudness adjustment is made.

3,l52,224 Patented Get. 6, 1964 It is, therefore, an object of thisinvention to overcome the aforenc-ted deficiencies of prior stereoloudness controls.

It is another object of this invention to provide a novel loudnesscontrol for use in stereophonic receivers.

It is a further object of this invention to provide a loudness controlarrangement which eliminates tracking error associated withsimultaneously actuated loudness controls.

It is an additional object to provide a new and improved loudnesscontrol for stereo receivers which is inexpensive contrasted to priorloudness controls.

The invention is used in a receiver for receiving stereo broadcastsignals comprising a single carrier concurrently frequency modulatedwith the sum of A and B program signals, a suppressed-carrieramplitude-modulated subcarrier signal carrying the difference of the Aand B signals, and a pilot signal related in frequency to thesubcarr-ier signal. The receiver comprises a frequencymodulationdetector responsive to the received carrier for developing a compositesignal corresponding to the modulation components and means responsiveto the pilot signal for deriving a modulation signal having a frequencycorresponding to that of the subcarrier. Additionally, the receivercomprises a synchronous stereo detector network and a potentiometercoupled between the frequencymodulation detector and the stereo detectornetwork for applying one polarity of e composite signal to the stereodetector network. Furthermore, the receiver includes means for applyingthe demodulation signal to the stereo detector network and a phaseinvertor network, coupled between the potentiometer and the stereodetector network, for applying the other polarity of the compositesignal to the stereo detector network in phase opposition to the outputsignals to the stereo detector network to effect matrixing and developcleanly separated A and B audio signals.

The features of this invention which are believed to be novel are setforth with particularity in the appended claims. The invention, togetherwith further objects and advantages thereof, may best be understood,however, by reference to the following description taken in conjunctionwith the accompanying drawings, in the several figures of which likereference numerals identify like elements, and in which:

FIGURE 1 is a schematic diagram of a stereo broad cast receiverembodying the loudness control apparatus of the invention; and

FIGURE 2 is a schematic diagram of an alternative embodiment of theapparatus of FIGURE 1.

The invention will be described with reference to the recently approvedstandards of the Federal Communications Commission for stereophonicbroadcasting. In accordance with the standards a single carrier isconcurrently frequency-modulated with: (1) the sum of the left and rightprogram signals, hereinafter designated as A and B; (2) the fundamentalcomponents of a suppressed-carrier amplitude-modulated sub-carriersignal carrying the difference of the A and B signals, where theexpression fundamental components means the first order modulationsidebands which attend the fundamental of the sub-carrier and excludesthe higher order sidebands attendant the harmonics of the sub-carriersignal; and (3) a pilot signal of a frequency corresponding to one-halfthe frequency of that sub-carrier. Since the subject invention concernsitself more particularly with the receiver, the preferred forms of thetransmitter for developing such a signal need not be illustrated nordescribed; however, illustrative transmitters are disclosed in detail inthe copending application of Robert Adler et 211., Serial No. 22,926,filed April 18, 1960 and also in the copending application of Carl G.Eilers, Serial No.

23,030, filed April 18, 1969, both assigned to the same assignee as thepresent application. 'Moreover, while the receiver is to be described inconjunction with the aforementioned FM stereophonic transmission system,the invention is broadly applicable to any single carrier stereophonictransmission system irrespective of the type of modulation. For example,the stereo transmission may employ amplitude-modulated sub-carriers,phase-modulated sub-carriers, an amplitudeand phase-modulated maincarrier, or any well known multiplex technique compatible with a singlemain carrier transmission system.

Referring now more particularly to FIGURE 1, the receiver there shownmay be employed for the reception of either monaural or stereophonicbroadcasts. The receiver adjusts itself between monophonic andstereophonic FM reproduction automatically in accordance with the typeof broadcast that is received. In presenting the structure and operationof this receiver, it is convenient to consider its use in thereproduction of stereophonic frequency-modulated broadcast signals ofthe type transmitted by apparatus described in the above-identifiedAdler et al. application.

The arrangement comprises receiver circuits which up to the secondsignal detector are conventional. They include a tunable radio-frequencyamplifier of any desired number of stages and a heterodyning stage orfirst detector, these being collectively represented by block 10. Theinput of the amplifying portion connects with a wave signal antenna 11.The output of block connects with a unit 12 which will be understood toinclude any desired number of stages of intermediate-frequencyamplification and one or more amplitude limiters.

The receiver is to have certain characteristics which are superior tothose found in conventional monophonic FM receivers. More specifically,it is preferred that the receiver have a high sensitivity so that thesignal-to-noise ratio, particularly on stereo operation, will beacceptable in fringe areas. Both automatic gain control for the RF andIF stages and automatic frequency control for the heterodyne oscillatorof unit 19 are desired and may be considered to have been included inthe block showing. The intermediate-frequency bandwidth of the usualmonaural FM receiver is 150 to 180 kilocycles (kc.) Wide at the minus 6decibel point but the bandwidth for the receiver under considerationshould be wider to prevent intermodulation or crosstalk between theseveral services that may be simultaneously accommodated on a singleradiation. A bandwidth of 230 kilocycles is adequate if automatic gaincontrol retains the level of signal through the RF and IF amplifiers ata substantially constant value in spite of variations in intensity ofthe received signal.

Following the IF amplifier and limiter 12 is a frequency-modulationdetector 15 responsive to the amplitude-limited intermediate-frequencysignal for demodulating a selected carrier signal to derive themodulation components thereof. Since effective amplitude limiting ishighly desirable in this receiver, it is convenient to follow thelimiter of unit 12 with the ratio detector 15 which also performs anamplitude limiting function. The composite signal detected infrequency-modulation detector 15 represents the complex modulation ofthe selected carrier Wave and is available at output terminals whichsupply the composite signal to various portions of the receiver. One setof output terminals P and P supply the composite signal to a pilotamplifier network 15.

It is necessary for stereo reproduction to have a further demodulationof the sub-carrier which conveys the difference information of the A andB signals. Since the sub-carrier is transmitted with no carriercomponent, the receiver has means responsive to the pilot signal of thereceived transmission for deriving a demodulation signal having a fixedfrequency and phase relation to the carrier component of thesuppressed-carrier amplitudemodulated sub-carrier. This means includesamplifier 16 which is tuned to the pilot frequency.

Pilot amplifier 16 in turn is coupled to a frequency doubler 17. Thefrequency doubler, when excited by the pilot signal, produces a signalof the same frequency as, and with a fixed phase relation to, thefundamental component of the carrier conveying the difierenceinformation. Suificient control of this phase may be provided byvariable tuning of one or more of the tuned circuits included in thepilot amplifier and frequency doubler chain.

A carrier amplifier 18 receives the signal from doubler 17 and amplifiesit to a magnitude sufiicient to energize a synchronous stereo detector19 which is coupled to the output of amplifier 18. The stereo detector19 serves as stereo-program-signal-developing means for producing the Aand B audio program signals The composite signal developed in the ratiodetector is fed from an output terminal D by Way of a potentiometer 30to synchronous detector 19. The potentiometer 30 is coupled betweenterminal D and ground and its wiper serves as adjustable means, coupledbetween detector 15 and stereo-program-signal-developing means 19, forvarying the magnitude of the signal presented to thestereoprograrn-signal-developing means. The demodulation signal obtainedfrom amplifier 18 and the composite signal obtained from detector 15control the diodes of detector 19 to the end that one diode developspredominantly the A audio signal with a slight contribution of the Baudio signal while the remaining diode develops principally the B audiosignal with a slight contribution of the A audio signal.

A clean separation of the A and B audio signals is obtained by matrixingin network 29 with the composite signal obtained from detector 15 by wayof potentiometer 3i and a phase inverter 15a. The addition of thisopposed phase signal and proper adjustment of the matrixing networkproduce a substantially pure A signal at terminal output a of the matrixand a substantially .pure B output terminal b. The matrixing network 29concurrently serves as a low pass filter for removing super-audiblecomponents from the detector output also perfiorms the desired functionof de-emphasis. The detecting and matrixing operation is explained morefully in the copending application of Adrian DeVries, Serial No. 118,-009, filed June 16, 1961, and assigned to the same assignee as thepresent application.

A potentiometer 31 forming a portion of a balance control, is connectedbetween terminal a and ground while its Wiper translates the A signal toan A audio amplifier 32 having a loudspeaker system 33 coupled thereto.Similarly, a potentiometer 35, forming the remaining portion of thebalance control, is coupled between terminal b and ground with its wiperconnected to a B audio amplifier 36 which in turn drives a loudspeakersystem 37. The balance control potentiometers 31 and 35 have their Wipercontacts respectively coupled together so that when actuated anincreased audio signal is presented to one audio amplifier while acorrespondingly decreased audio signal is applied to the remaining audioamplifier. This construction and operation is well known in the art.Alternatively, one potentiometer may be eliminated and balance achievedby adjustment of the remaining potentiometer but this is undesirable inthat the loudness control must be adjusted after a balance adjustment ismade.

Loudness is adjusted by means of potentiometer 30. The setting ofpotentiometer 39 determines the magnitude of the composite signalpresented to synchronous detector 19. As the magnitude of this signal isvaried, the magnitude of the A and B audio signals is likewise varied.Varying the magnitude of the composite signal presented to stereodetector 19 affects the A and B modulation components equally and,consequently, tracking errors experienced with previous loudnesscontrols are eliminated.

Once the balance level of the reproduced program has been set withbalance controls 31, 35, loudness control 30 may be adjusted at willwithout disturbing the balance.

FIGURE 2 shows an alternative embodiment of the invention wherein thewiper of potentiometer 30 is coupled to a filter network 40 whichseparates at least the A+B signal and the (AB)S signal. The (AB)S signalis applied to a demodulator 41 of conventional construction and ademodulation signal which may be derived from the received pilot is alsodelivered to demodulator 41 which develops an AB output signal. A matrixnetwork 42 is coupled to both filter 46 and to demodulator 41 andreceives the A-l-B and A-B signals respectively therefrom. The matrixcombines these signals in conventional fashion and provides A and Boutput signals at terminals a and b which correspond to terminals a andb of FIGURE 1. As the single control 30 is used to adjust the magnitudeof the A and B components equally, the correct relationship between theA and B signals at the output of matrix 42 will be maintained.

Thus the invention provides a new and improved loudness control forstereophonic broadcast receivers. The

customary concentrically mounted, simultaneously actuated potentiometersnormally employed as a loudness control have been eliminated andreplaced by an inexpensive loudness control which does not have trackingerrors and does not adversely afiect balance.

While particular embodiments of the present invention have been shownand described, it is apparent that changes and modifications may be madetherein without departing irom the invention in its broader aspects. Theaim of the appended claim, therefore, is to cover all such changes andmodifications as fall within the true spirit and scope of the invention.

I claim:

A receiver for a stereophonic frequency-modulation system for utilizinga transmitted signal comprising a single carrier concurrentlyfrequency-modulated with the sum of A and B program signals, asuppressed-carrier amplitude-modulated subcarrier signal carrying thedifierence of the A and B signals, and a pilot signal related infrequency to the subcarrier signal, said receiver comprising:

a frequency-modulation detector responsive to said received carrier fordeveloping a composite signal corresponding to said modulationcomponents;

means responsive to said pilot signal for deriving a demodulation signalhaving a frequency corresponding to that of said subcarrier;

a synchronous stereo detector network;

a potentiometer coupled between said frequency modulation-detector andsaid stereo detector network for applying one polarity of said compositesignal to said stereo detector network;

means for applying said demodulation signal to said stereo detectornetwork;

and a phase inverter network, coupled between said potentiometer andsaid stero detector network, for applying the other polarity of saidcomposite signal to said stereo detector network in phase opposition tothe output signals of said stereo detector network to effect matrixingand develop cleanly separated A and B audio signals.

References Cited in the file of this patent UNITED STATES PATENTS2,515,619 Weyers July 18, 1950 3,070,662 Eilers Dec. 25, 1962 3,087,994Schutte Apr. 30, 1963

1. A RECEIVER FOR A STEREOPHONIC FREQUENCY-MODULATION SYSTEM FORUTILIZING A TRANSMITTED SIGNAL COMPRISING A SINGLE CARRIER CONCURRENTLYFREQUENCY-MODULATED WITH THE SUM OF A AND B PROGRAM SIGNALS, ASUPPRESSED-CARRIER AMPLITUDE-MODULATED SUBCARRIER SIGNAL CARRYING THEDIFFERENCE OF THE A AND B SIGNALS, AND A PILOT SIGNAL RELATED INFREQUENCY TO THE SUBCARRIER SIGNAL, SAID RECEIVER COMPRISING: AFREQUENCY-MODULATION DETECTOR RESPONSIVE TO SAID RECEIVED CARRIER FORDEVELOPING A COMPOSITE SIGNAL CORRESPONDING TO SAID MODULATIONCOMPONENTS; MEANS RESPONSIVE TO SAID PILOT SIGNAL FOR DERIVING ADEMODULATION SIGNAL HAVING A FREQUENCY CORRESPONDING TO THAT OF SAIDSUBCARRIER; A SYNCHRONOUS STERO DETECTOR NETWORK; A POTENTIOMETERCOUPLED BETWEEN SAID FREQUENCY MODULATION-DETECTOR AND SAID STEREODETECTOR NETWORK FOR APPLYING ONE POLARITY OF SAID COMPOSITE SIGNAL TOSAID STEREO DETECTOR NETWORK;